[RFC] PPC: MPIC: necessary readback after EOI?

[Purcareata Bogdan]

Hello,

While doing some performance testing of a KVM guest on a PPC platform, I 
noticed that there's a read of the CPU_WHOAMI register after each MPIC 
EOI [1]. This has been present since the initial implementation of the 
MPIC driver [2]. In a KVM virtualized environment, this results in an 
additional kvm_exit.

Is the read back necessary? Is it used to provide some sort of 
synchronization mechanism, making sure that nothing else is executed 
until the EOI write is finished? I eliminated the mpic_cpu_read call and 
run the kernel on hardware and noticed no anomaly, however I am not sure 
of all the implications and race conditions it might lead to.

I was curious why the mpic_cpu_read(MPIC_INFO(CPU_WHOAMI)) was there in 
the first place and if it's still needed. If it's still required, I 
guess a better approach is to eliminate the call only if the kernel is 
running on the KVM guest side, where the MPIC is emulated and no longer 
requires a readback.

Thank you,
Bogdan P.

[1] http://lxr.free-electrons.com/source/arch/powerpc/sysdev/mpic.c#L659
[2] https://lkml.org/lkml/2004/10/22/483